Trip valves which are latched manually and armed by the operation of hydraulic pressure and which thereby monitor the operation of a pipeline are known. Such trip valves, when tripped by the action of a change in pressure in a pilot circuit, are operable to allow the exit of relatively large amounts of hydraulic monitoring fluid which may be necessary to shut down a valve in a pipeline used to terminate flow in the pipeline.
Such valves, however, are often of the "spool" variety. That is, the sealing relationships in known latching trip valves are caused by metal to metal "rubbing" contact. While metal to metal contact can create and maintain suitable seals, leakage is still frequently a problem under various conditions. In addition, such metals are necessarily hard in order to reduce friction between the metal surfaces and in order that impurities in the hydraulic oil within the trip valve will not score or damage the metal surfaces. Such metals are expensive and, although being hard, still remain subject to scoring and unsatisfactory operation with the passage of time.